Paracrine Activation of Hepatic CB1 Receptors by Stellate Cell-Derived Endocannabinoids Mediates Alcoholic Fatty Liver

Abstract

Alcohol-induced fatty liver, a major cause of morbidity, has been attributed to enhanced hepatic lipogenesis and decreased fat clearance of unknown mechanism. Here we report that the steatosis induced in mice by a low-fat, liquid ethanol diet is attenuated by concurrent blockade of cannabinoid CB1 receptors. Global or hepatocyte-specific CB1 knockout mice are resistant to ethanol-induced steatosis and increases in lipogenic gene expression and have increased carnitine palmitoyltransferase 1 activity, which, unlike in controls, is not reduced by ethanol treatment. Ethanol feeding increases the hepatic expression of CB1 receptors and upregulates the endocannabinoid 2-arachidonoylglycerol (2-AG) and its biosynthetic enzyme diacylglycerol lipase beta selectively in hepatic stellate cells. In control but not CB1 receptor-deficient hepatocytes, coculture with stellate cells from ethanol-fed mice results in upregulation of CB1 receptors and lipogenic gene expression. We conclude that paracrine activation of hepatic CB1 receptors by stellate cell-derived 2-AG mediates ethanol-induced steatosis through increasing lipogenesis and decreasing fatty acid oxidation.

Transcriptional Regulation of Cannabinoid Receptor-1 Expression in the Liver by Retinoic Acid Acting via Retinoic Acid Receptor-γ

“…In mice fed a liquid alcohol diet, the development of fatty liver was found to involve paracrine activation of hepatic CB 1 R by hepatic stellate cell (HSC)-derived endocannabinoids. Chronic alcohol feeding also resulted in up-regulation of CB 1 R in hepatocytes in vivo, which could be replicated in vitro by co-culturing control mouse hepatocytes with HSC isolated from ethanol-fed mice (1). This suggests that HSC-derived mediator(s) can regulate CB 1 R expression in hepatocytes.…”

“…CB 1 ϩ/ϩ and CB 1 Ϫ/Ϫ littermates were obtained by breeding heterozygotes that had been back-crossed to a C57BL/6J background, as described (13). Mice with hepatocyte-specific knock-out of CB 1 R (LCB 1 Ϫ/Ϫ mice) were generated as described (1). Raldh1 Ϫ/Ϫ mice on a mixed 129/C57Bl/6 background were generated as described (14).…”

“…Binding of RAR␥ to the CB 1 R Gene Promoter-In view of the observed regulation of CB 1 R by RAR␥, we tested whether RAR␥ can bind to the CB 1 R promoter. A CB 1 R promoter fragment was generated by PCR using biotinylated primers spanning a 500-bp 5Ј upstream region of the CB 1 R gene.…”

“…Recent findings indicate that CB 1 R are expressed at low yet functionally relevant levels in the liver, and their activation promotes de novo lipogenesis and inhibits fatty acid oxidation (3). Through these effects, endocannabinoids play a key role in the development of fatty liver in response to high fat diets (4) or chronic alcohol intake (1). In mice fed a liquid alcohol diet, the development of fatty liver was found to involve paracrine activation of hepatic CB 1 R by hepatic stellate cell (HSC)-derived endocannabinoids.…”

“…1 To whom correspondence may be addressed: National scription factors. Receptors for RA consist of heterodimers of RAR and retinoid X receptors (RXR).…”